The present invention relates generally to the art of sputtering transparent metallic films for solar energy control, and more particularly to the art of controlling the color and durability of sputtered films.
Transparent films of metals such as silver, copper, nickel and cobalt are readily produced by electroless deposition methods. U.S. Pat. No. 3,457,138 to Miller discloses an electroless (wet chemical) method for producing transparent copper films over a thin transparent silver film on glass. Similarly produced wet chemical silver/copper films on glass have achieved commercial success as architectural products for solar energy control. The coating effectively reflects solar heat energy while presenting an aesthetically appealling low reflectance pinkish colored surface to the observer.
However, advances in coating technology provide alternative processes for producing transparent metallic films for solar energy control, such as chemical vapor deposition, vacuum evaporation deposition, and cathode sputtering. Cathode sputtering is particularly desirable for producing metallic films in a method that is fast and efficient on a large scale. U.S. Pat. No. 4,166,018 to Chapin discloses a particularly useful sputtering process and apparatus which employ a magnetic field to improve cathode sputtering conditions. A sputtering apparatus is described in which a magnetic field is formed adjacent a planar sputtering surface. The field comprises arching lines of flux over a closed loop erosion region of the cathode surface.
In attempting to reproduce the properties of a wet chemical two-layer silver/copper film on glass by cathode sputtering, using known cathode sputtering techniques to produce two-layer silver/copper films at various thicknesses and thickness ratios, the desirable low reflectance pinkish colored appearance of the wet chemical two-layer silver/copper film is not obtained. Rather, typical sputtering conditions result in two-layer silver/copper films that range in appearance from highly reflective bright silver to highly reflective bright copper depending on the silver/copper ratio and the orientation of the sample.
U.S. Pat. No. 4,462,884 to Gillery et al entitled "Low Reflectance, Low Emissivity Sputtered Films" discloses a method for producing transparent two-layer silver/copper films having a low reflectance pinkish colored appearance by cathode sputtering. The method disclosed therein requires the deposition of a first layer of silver in a discontinuous agglomerated state. The deposition of a continuous copper film over the discontinuous agglomerated silver film produces the desired low reflectance pinkish colored appearance characteristic of the wet chemical product. The deposition of silver in a discontinuous agglomerated state by cathode sputtering requires conditions which allow the silver arriving at the substrate surface sufficient time to agglomerate. This can be accomplished by either slowing the arrival rate of silver or increasing its mobility at the substrate surface. Slowing the arrival rate of the sputtered silver may be achieved by either lowering the power applied to the sputtering cathode or by increasing the gas pressure in the vacuum chamber in which the sputtering process is being performed. Increasing the mobility of the silver may be achieved by raising the temperature or decreasing the surface energy of the substrate.
Unfortunately, the discontinuous silver layer applied by cathode sputtering as described above does not have as good adhesion to glass as is desired for a commercial window unit. Moreover, the degree of agglomeration of the silver is difficult to control. The deposition of the silver layer is affected by the condition of the glass surface and the method used to clean it. The color and adhesion of a dual layer coating of silver and copper are particularly difficult to optimize if the glass substrate has been in storage and has stain or incipient stain on the surface to be coated.
U.S. Pat. No. 4,512,863 to Criss et al provides a method for achieving improved adhesion and color control of a sputtered film of a metal such as silver or copper by means of a primer layer of a metal such as stainless steel. The sputtering of a stainless steel primer layer is particularly useful in the production of a desirable silver/copper two-layer sputtered film on glass. The stainless steel primer layer may be sputtered onto a substrate surface to a thickness on the order of an atomic layer to provide a suitable surface for the sputtering of more sensitive metals such as silver. Thicker primer layers are also disclosed, limited only by the desired final transmittance of the coated article. Two-layer silver/copper films are preferably stabilized by treatment with passivating agents.